In the vein of Gary Ross’s 2003 “Seabiscuit,” which celebrated the undersized racehorse that lifted American spirits during the Great Depression, comes Louise Osmond’s “Dark Horse,” which chronicles how an unheralded British thoroughbred’s unlikely track success represented the triumph of plucky commoners over the swells in a sport dominated by wealth and social privilege. (Actually there was an American movie that covered similar territory—2014’s “50 to 1,” about Mine That Bird, the unexpected winner of the 2009 Kentucky Derby. But virtually nobody saw it.) Unlike that movie or “Seabiscuit,” however, this is a documentary, a combination of archival footage, home movies and interviews newly shot by Benjamin Kracun, knitted together by editor Joby Gee to make for a crowd-pleasing tale of winning against all odds that’s practically impossible not to like.

The story has its geographical heart in Caerphilly, a village in South Wales suffering hard economic times after mine closures. Jan Vokes, a barmaid at a local pub who’d previously had experience breeding dogs and pigeons, overheard a customer, Howard Davies, reminiscing about how he’d once owned a racehorse—and lost a bundle in the process, to the distress of his wife Angela. That inspired Jan, who, together with her husband Brian, purchased a mare called Rewbell at a bargain price and bred the horse (again at minimal cost) with a stallion called Bien Bien. In 2001 the foal was born—an event captured on film—and raised in Cefn Fforest outside town. Jan enlisted Davies to serve as a manager, and he formed what came to be known as the Syndicate, a group of local residents who each contributed a nominal sum each week to finance the training of the horse, which they named Dream Alliance. The nest egg was sufficient to put Dream in the care of professional trainer Philip Hobbs, and by 2004 he was ready to race, eventually winning the Perth Gold Cup in 2007.

All this is told through film clips and interviews with the Vokeses, Howard and Angela Davies, Hobbs and undertrainer Johnson White, and investors including Maureen Jones and Tony Kerby. Together they’re a colorful, immensely likable group who recall their astonishment at Dream’s success and their feelings of out-of-placeness among the well-to-do owners who dominated the sport. The unusual circumstances brought the horse and them a good deal of media attention, which only escalated when Dream was seriously injured in a 2008 run-up race to the Grand National that might have ended his life. But the investors, whose emotional attachment was greater than their love of profit, put their winnings into innovative stem-cell surgery that proved successful, leading to his recovery and eventual victory in the 2009 Welsh National and anticipation about how he would fare in the 2010 Grand National.

What transpires from that point has to be left to the reader to find out, but the point of “Dark Horse” isn’t so much the awards and prize money that might follow, but the bond that grew over the years between Dream Alliance the members of the Syndicate, who actually choose the interests of the horse over their own in instance after instance. The love of the animal exhibited by Jan and Brian, as well as Jones and Kerby, is palpable, and even Hobbs, an outsider of sorts, tears up as he recalls working with the horse, who does seem, from the extended footage of him, to be both beautiful and—with a nod in the direction of anthropomorphism—immensely personable.

“Dark Horse” plays on the class consciousness that continues to be a major element of British society, but as recent US history demonstrates, that’s a reality which is hardly alien to the American experience as well, and while “50 to 1” proved incapable of capitalizing on it, Osmond’s effort might just fare better, at least with the arthouse crowd. Of course, eventually it could spawn a dramatization along the lines of “Seabiscuit.” Such a project would be perfect for the revived Ealing Studios—a modernized version of the “little guy” movies like “Passport to Pimlico” and “The Titfield Thunderbolt” that were a specialty of theirs in the forties and fifties. And though critics shouldn’t be too free with casting suggestions, one has to say that if an Ealing “Dream Alliance” should get greenlit, Kerby would just have to be played by Jim Broadbent.